A radio frequency carrier may be generated by using a local oscillator operating at the carrier frequency or by using two local oscillators and a mixer to generate the desired carrier frequency as the sum of the two local oscillator frequencies. The first scheme is sometimes referred to as "direct" up-conversion while the latter scheme is sometimes referred to as "double" up-conversion. In double up-conversion one of the local oscillators generates an intermediate frequency (IF) and the other local oscillator generates a frequency which is that of the radio frequency (RF) carrier minus the IF frequency. The mixer generates both sum and difference frequencies. The unwanted frequency, usually the difference frequency, is filtered out.
In either of the conversion arrangements, single sideband suppressed carrier operation requires that the baseband signal be derived from balanced amplifers and applied to two mixers in phase quadrature so that the carrier may be suppressed. After modulation, the desired RF sideband signal is power amplified and transmitted from an antenna. Unwanted sidebands are attenuated by suitable filters. Because the RF power is large compared to the power of the local oscillator, a small fraction of the signal from the antenna fed back to the local oscillator may have sufficient strength to cause the local oscillator to malfunction. In hand-held radios at 900 mHz, for example, sufficient shielding is almost impossible to achieve. Moreover, single sideband suppressed carrier systems dictate that the carrier frequency power level be at least 30 Db down from the power level of the desired sideband. Any leakage of the local oscillator to the output of the modulator package must therefore be controlled. Such leakage may be troublesome if, when the output power is desired to be attenuated, the leakage power remains constant.
In addition, a quadrature coding scheme requires that 90 degree phase-shifted signals be accurately generated. R-C networks for achieving an accurate 90 degree phase shift are difficult to achieve with the usual production run accuracy available in integrated circuits. Accordingly, such R-C networks must be fabricated as off-chip devices so that component tolerances can more accurately be controlled. It would be desirable to provide a modulation arrangement requiring only one local oscillator with all components being capable of being implemented on a single chip.